DVD disks are achieving widespread use as recording media for storing multimedia data including a moving image, and audio data. Various kinds of formats for the DVD disks have been proposed thus far.
In particular, in a commonly-used physical format of writing type DVD disks, the disk surface is sectioned into groove areas (referred to as “groove”) and land areas (referred to as “land”), each of which is located between grooves. Addressing is performed by meandering (wobble) of the grooves.
In the case of DVD-R/DVD-RW that is a kind of format of writing type DVD disks, in addition to the wobble, each land area is provided with pits used for addressing (each pit is called a land prepit (LPP)) by which address information on the optical disk is recorded.
JP-A-2004-134002 discloses the technology for detecting LPP by a push-pull signal obtained by a photodetector. The LPP detection method disclosed in JP-A-2004-134002 includes the steps of: holding as a peak S1 an amplitude fluctuation component that is caused by noises and meandering grooves; adding an offset component S2 to the peak S1 to generate comparison reference voltage Vth; and detecting LPP by comparing a push-pull signal with the comparison reference voltage Vth. JP-A-2004-134002 states that the above-described method makes it possible to stably detect LPP without being influenced by amplitude fluctuations caused by noises and recording marks.
In addition, JP-A-2005-166211 discloses a LPP detection technology including the steps of: dividing the difference between a peak value of a prepit detection circuit and a peak value of a wobble signal component; and using the divided voltage level as a slice level to detect LPP by a comparator.
Problems with the LPP detection circuit according to the prior art will be described with reference to FIG. 7 and FIGS. 8A and 8B as below.
FIG. 7 is a circuit diagram illustrating the LPP detection circuit according to the prior art.
FIGS. 8A and 8B are waveform charts each illustrating LPP detection according to the prior art.
A circuit shown in FIG. 7 is based on the same idea as that of JP-A-2005-166211. To be more specific, a voltage value indicated by two peak hold circuits is divided to determine a slice level (threshold value) used for the LPP detection.
A LPP detection circuit 0 is a circuit for inputting a push-pull signal and for outputting a signal used to detect LPP, the push-pull signal being generated by amplifying the output of a photodetector (not illustrated).
As shown in FIG. 7, the LPP detection circuit 0 of the optical disk drive according to the prior art to be described hereinbelow includes a peak hold circuit 1:10, a LPF (Low Pass Filter) 20, a peak hold circuit 2:30, a slice level generation circuit 40, and a comparator 50.
A push-pull signal that has been input into the LPP detection circuit 0 is input into the peak hold circuit 1:10, the LPF 20, and the peak hold circuit 2:30.
The peak hold circuit 1:10 outputs a peak voltage P1 of the inputted voltage. On the other hand, the LPF 20 attenuates the inputted signal. The attenuated signal is then input into the peak hold circuit 2:30. After that, a peak voltage P2 is output from the peak hold circuit 2:30. Next, the peak voltage P1 output from the peak hold circuit 1:10 and the peak voltage P2 output from the peak hold circuit 2:30 are input into the slice level generation circuit 40. Then, the voltage difference between the peak voltage P1 and the peak voltage P2 is divided by a constant ratio to generate a slice level Vs1.
To be more specific, the slice level Vs1 is expressed by equation 1 as follows:Vs1=r(P1−P2)  (Equation 1)where r is a proper division ratio used to detect LPP. For example, r is 1/2=50%.
Next, the comparator 50 compares the slice level Vs1, which has been generated by the slice level generation circuit 40, with the push-pull signal. If the push-pull signal is larger than Vs1, it is to be judged that LPP have been detected.
During a period (A) shown in FIG. 8A, the push-pull signal output at a position of LPP is intermittently shown. In response to a push-pull signal which has become larger than the slice level Vs1, LPP is detected as shown in FIG. 8B.
However, if an extremely high spike-shaped peak voltage P1 occurs due to a noise n generated by a land area defect, Vs1 increases. Accordingly, as indicated by (B) in FIG. 8A, there occurs a case where until the slice level Vs1 decreases to a level (C) shown in FIG. 8A, LPP is not detected although the LPP exists.